Writing code to verify that a date is valid $($exists within the calendar$)$ as a module rather than part of a main program is advantageous because

$$ a$.$ placing code within a module increases its reliability

$$ b$.$ the many programs that require date validation can then call this same module rather than implement it independently

$$ c$.$ the module statements can then be copied automatically into each main program that requires date validation by calling the module

$$ d$.$ code in modules achieves a greater level of abstraction compared to code in a main program